This invention relates to an imaging medium with a bubble-suppressant layer, and to an imaging process using such an imaging medium.
Imaging media are known which have at least one color-forming layer comprising a color-forming composition adapted to undergo a change of color (from colorless to colored, from colored to colorless, or from one color to another) upon increase in the temperature of the color-forming layer above a color-forming temperature for a color-forming time. The color change in such media need not be supplied by applying heat directly to the medium; the color-forming composition may comprise a color-forming compound which undergoes a change of color upon heating above a color-forming temperature, and an absorber capable of absorbing actinic radiation and thereby generating heat in the color-forming layer. When such a medium is exposed to appropriate actinic radiation, this radiation is absorbed by the absorber, thereby heating the color-forming compound and causing it to undergo its color change. Many such thermal imaging media have the advantage over conventional silver halide media of not requiring a post-exposure developing step. Such thermal imaging media also have the advantage that they are essentially insensitive to visible light, so that they can be handled under normal lighting conditions.
For example U.S. Pat. Nos. 4,602,263 and 4,826,976 both describe thermal imaging systems for optical recording and particularly for forming color images. These thermal imaging systems rely upon the irreversible unimolecular fragmentation of one or more thermally unstable carbamate moieties of an organic compound to effect a visually discernible color shift. U.S. Pat. No. 4,720,449 describes a similar imaging system in which the color-developing component is a substantially colorless di- or triarylmethane imaging compound possessing within its di- or triarylmethane structure an aryl group substituted in the ortho position to the meso carbon atom with a moiety ring-closed on the meso carbon atom to form a 5- or 6-membered ring, said moiety possessing a nitrogen atom bonded directly to the meso carbon atom and the nitrogen atom being bound to a group with a masked acyl substituent that undergoes fragmentation upon heating to liberate the acyl group for effecting intramolecular acylation of the nitrogen atom to form a new group in the ortho position that cannot bond to the meso carbon atom, whereby the di- or triarylmethane compound is rendered colored. Other thermal imaging systems using di- or triarylmethane compounds are described in U.S. Pat. Nos. 4,720,450 and 4,960,901, while U.S. Pat. No. 4,745,046 describes a thermal imaging system using as color-forming co-reactants a substantially colorless di- or triarylmethane compound possessing on the meso carbon atom within its di- or triarylmethane structure an aryl group substituted in the ortho position with a nucleophilic moiety which is ring-closed on the meso carbon atom, and an electrophilic reagent which upon heating and contacting the di- or triarylmethane compound undergoes a bimolecular nucleophilic substitution reaction with the nucleophilic moiety to form a colored, ring-opened di- or triarylmethane compound. Finally, the aforementioned U.S. Pat. No. 5,192,645 describes a thermal imaging system in which the color-forming component is a colorless precursor of a preformed image dye substituted with (a) at least one thermally removable protecting group that undergoes fragmentation from the precursor upon heating and (b) at least one leaving group that is irreversibly eliminated from the precursor upon heating, provided that neither the protecting group nor the leaving group is hydrogen, said protecting and leaving groups maintaining the precursor in its colorless form until heat is applied to effect removal of the protecting and leaving groups, whereby the colorless precursor is converted to an image dye.
The aforementioned patents describe a preferred form of imaging medium for forming multicolor images; in this preferred imaging medium, three separate color-forming layers, capable of forming yellow, cyan and magenta dyes respectively, are superposed on top of one another. Each of the three color-forming layers has an infra-red absorber associated therewith, these absorbers absorbing at differing wavelengths, for example 760, 820 and 880 nm. This medium is imagewise exposed simultaneously to three lasers having wavelengths of 760, 820 and 880 nm. (In the present state of technology, solid state diode lasers emitting at about 760 to 1000 nm provide the highest output per unit cost. Since most of the color-forming materials (also hereinafter referred to as "leuco dyes", with the understanding that the leuco dye may comprise more than one compound) described in the aforementioned patents do not have high extinction coefficients within this wavelength range, it is necessary to include the infra-red absorbers with the leuco dyes in order to ensure efficient absorption of the laser radiation and hence efficient heating of the leuco dye.) The resultant imagewise heating of the color-forming layers causes the leuco dyes to undergo color changes in the exposed areas, thereby producing a multicolored image, which needs no development.
This preferred type of imaging medium is capable of very high resolution images; for example, the medium can readily be used to produce a 2000 line 35 mm slide. However, it has now been found that, when this preferred type of imaging medium is used to produce a slide or other transparency, strongly colored areas of the image which appear to have the correct color when viewed in reflection against a white background appear essentially black when the image is projected (i.e., seen in transmission). This discrepancy between the appearance of the image in reflection and transmission will hereinafter be referred to as "blackening" of the image. The discrepancy can be dramatic; the present inventors have produced images with areas which appear chrome yellow in reflection but black in transmission.
It has now been found that this blackening of the image is due to the formation of bubbles in the color-forming layer(s) and can be reduced or eliminated by providing the imaging medium with a bubble-suppressant layer of appropriate thickness. (The term "bubbles" is used herein to refer to bubbles, voids, cracks, tears and similar artifacts which are present in the final image and which scatter visible light.)